The invention relates to a method for the catalytic conversion of nitrogen oxides contained in the exhaust gas of an internal combustion engine. A reducing agent is added to the exhaust gas upstream of a denitrification catalytic converter, as seen in exhaust gas flow direction, and is metered as a function of a nitrogen oxide concentration.
During the selective catalytic reduction (SCR) of nitrogen oxides, a reducing agent, which is generally ammonia or a substance that releases ammonia, for example an aqueous urea solution, has to be added to an exhaust gas which contains the nitrogen oxides. The reducing agent is converted, together with the nitrogen oxides, at an SCR catalytic converter, which is generally based on titanium oxide with additions of metal oxides such as, for example, vanadium, molybdenum and/or tungsten oxide, to form environmentally harmless nitrogen, oxygen, and water.
When the exhaust gas generator (engine, furnace) is operated under non-steady-state conditions, as is the case in particular with vehicle engines, the nitrogen oxide concentrations, the exhaust gas mass flow rate and the exhaust gas temperature vary within wide ranges. This fact places considerable demands on the metering strategy and metering accuracy for the reducing agent which is introduced into the exhaust gas.
In most currently known methods for metering the reducing agent, the reducing agent is introduced into the exhaust gas proportionally to the mass flow of nitrogen oxides. In that case, the flow of reducing agent which is supplied to the exhaust gas has to be selected in significantly substoichiometric proportions in relation to the nitrogen oxide concentration, in order to reliably prevent a breakthrough of the reducing agent, in particular an ammonia breakthrough. However, that has the drawback of not completely utilizing the catalytic activity of the catalytic converter, and consequently higher conversion rates, which can in principle be achieved with the catalytic converter, cannot be achieved due to that metering strategy.
It is known from German Published, Non-Prosecuted Patent Application DE 37 21 572 A1 to use the power and rotational speed of the engine for metering of the reducing agent.
Moreover, it is known from European Patent Application 0 555 746 A1 for the metering of reducing agent to be interrupted completely in the event of the exhaust gas temperature falling below a predetermined limit value and for the metering to be recommenced as a function of the load and rotational speed of the internal combustion engine, only once the temperature limit value has been reached again.
Furthermore, European Patent Application 0 515 857 A1 has disclosed a method in which the reducing agent is metered in a pulsed manner in superstoichiometric proportions and the catalytic converter is run xe2x80x9cemptyxe2x80x9d in intervals between two pulses. In that method, the cyclical addition of superstoichiometric proportions of reducing agent is only interrupted again when a high NH3 concentration, which has been set as a threshold, in the gas phase has been reached at a certain point in a catalyst bed.
European Patent Application 0 362 483 A1 has disclosed a pulse-width-modulated discontinuous metering method, in which the pulse width is set as a function of the exhaust gas mass flow rate and the nitrogen oxide concentration.
The latter four methods are also afflicted with the drawback described above.
Furthermore, International Publication No. WO 96/04980, corresponding to U.S. Pat. No. 5,785,937, has disclosed a method in which the reducing agent is only metered in superstoichiometric proportions, with respect to the nitrogen oxide concentration, during the starting phase of the internal combustion engine and during operation with a falling and if appropriate also a virtually constant exhaust-gas temperature, while taking the temperature-dependent storage capacity of the catalytic converter into account. Otherwise, the reducing agent is metered in substoichiometric proportions.
That method too does not allow the full extent of the higher conversion rate which can be achieved in principle with the catalytic converter, to be reached.
It is accordingly an object of the invention to provide a method for the catalytic conversion of nitrogen oxides contained in the exhaust gas of an internal combustion engine, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods of this general type and which allows the catalytic activity of the catalytic converter to be utilized more successfully.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for the catalytic conversion of nitrogen oxides contained in the exhaust gas of an internal combustion engine, by adding a reducing agent to the exhaust gas upstream of a denitrification catalytic converter, as seen in exhaust gas flow direction, and metering the reducing agent as a function of a nitrogen oxide concentration. The method comprises measuring an operating parameter of the internal combustion engine influencing the metering; assigning a limit value to the operating parameter; and establishing an operating state of the internal combustion engine being unfavorable for metering, upon falling below or exceeding the limit value. A metered quantity of reducing agent is increased in a preset manner before and/or during falling below or exceeding the limit value. The increase in the metered quantity is terminated during the unfavorable operating state.
The method makes use of the ability of the catalytic converter to store reducing agent. If an SCR control unit which operates in accordance with the method detects the beginning of an operating state of the internal combustion engine which is unfavorable for the metering of reducing agent, the metered quantity is, for example, increased briefly. In this way reducing agent is stored in the catalytic converter. This reducing agent is then available for the nitrogen oxide reduction process during the operating state which is unfavorable for metering (interim storage).
The result is a higher mean nitrogen oxide conversion, in particular during fluctuating operating conditions of the internal combustion engine.
In accordance with another, extremely advantageous, mode of the invention, the variable derived from the operating parameter is derived by differentiating the operating parameter according to time or by forming the difference between measured values of the operating parameter taken at different times. A method which operates in this way is able to detect an operating state which is unfavorable for the metering of reducing agent and to do so particularly quickly and at a particularly early stage.
In accordance with a further mode of the invention, a constant quantity of reducing agent may be metered in the event of falling below or exceeding the particular limit value.
In accordance with an added mode of the invention, the quantity of reducing agent which is metered in superstoichiometric proportions is set as a function of the variable derived from the operating parameter by differentiation or the variable derived from the operating parameter by forming a difference. For example, a corresponding metering interval width may be dimensioned accordingly.
In accordance with an additional mode of the invention, the quantity of reducing agent metered in superstoichiometric proportions is set as a function of the extent to which the limit value of the operating parameter or the limit value of the variable derived from this parameter is exceeded or fallen below.
In accordance with yet another mode of the invention, the quantity of reducing agent metered in superstoichiometric proportions is set as a function of the storage capacity, which is in particular temperature-dependent, of the denitrification catalytic converter.
In accordance with yet a further mode of the invention, the value of the quantity of reducing agent metered in superstoichiometric proportions is stored. For example, an electronic storage medium may be used for this purpose.
The stored value is then taken into account during the further control of the metering of reducing agent. This is important in particular if, shortly after the superstoichiometric metering, contrary to expectation, there is not in fact a prolonged phase with unfavorable metering conditions, but rather good metering conditions are immediately restored. In this situation, then, the preferred procedure which has been described above makes it possible to prevent overfilling of the catalytic converter which stores reducing agent and slippage of reducing agent.
In accordance with a concomitant mode of the invention, the operating parameter is an exhaust gas temperature, an exhaust gas mass flow rate, a load of the internal combustion engine or a rotational speed of the internal combustion engine.
If the operating parameter is the exhaust gas temperature, it is particularly advantageous, in the event of falling below (or exceeding) a limit value of a variable derived from the exhaust-gas temperature by the formation of a differential quotient, for the superstoichiometric metering to be triggered. This is because temperature sensors are often very slow and would only reliably confirm falling below a limit value for the exhaust-gas temperature after a time delay. By contrast, when differentiating the temperature profile this event is almost xe2x80x9canticipatedxe2x80x9d, and an unfavorable operating state is detected very early.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for the catalytic conversion of nitrogen oxides contained in the exhaust gas of an internal combustion engine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.